1. Field of the Invention
The present invention relates to junction assemblies for use in magnetic head devices and the like, in which bonding pads provided on substrates, and bumps provided on wiring members and composed of copper or the like are bonded to each other by soldering. More particularly, the present invention relates to a junction assembly of a bonding pad having an Au film formed on the surface thereof.
2. Description of the Related Art
FIG. 10 is a cross-sectional view showing a conventional bonding pad, and FIG. 11 is a cross-sectional view showing a copper bump being soldered to the bonding pad.
The bonding pad 2 shown in FIG. 10 is formed by sequentially depositing a Cu film 2a, an Ni film 2b, and an Au film 2c on a base film la constituting a flexible substrate 1. Wettability in soldering is decreased when the Cu film 2a is oxidized; however, by forming the Ni film 2b, oxidation of the Cu film 2a can be prevented and the decrease in the wettability can be avoided. In addition, by forming the Au film 2c on the surface of the Ni film 2b, the wettability of the bonding pad to the solder can be further improved.
In the flexible substrate 1, a conductive pattern composed of a Cu film is formed on the base film 1a, and a part of the Cu film constituting the conductive pattern is used as the Cu film 2a of the bonding pad 2a. The surface of the conductive pattern, except for the bonding pad 2, is covered by an insulating layer, such as a resist.
A wiring member 3 shown in FIG. 11 has a conductive pattern 3b composed of copper foil formed between resin films 3a and 3a. A copper bump 3c protruding from gaps between the resin films 3a and 3a through an opening provided in one resin film 3a, is provided. The surface of the copper bump 3c is covered by a solder layer.
By contacting the copper bump 3c covered by the solder layer at the surface thereof with the bonding pad 2 and heating the wiring member 3 and the flexible substrate 1 having the bonding pad 2 formed thereon, which are sandwiched between a heater and a holding plate for heating, the solder on the surface of the copper bump 3c is melted. A solder junction portion 4 bonding the bonding pad 2 and the copper bump 3c is thus formed.
However, the conventional junction assembly has a problem in that bonding strength of the soldering junction 4 is not satisfactory.
The bonding strength of the soldering junction 4 can be measured by observing a position at which the wiring member 3 and the flexible substrate 1 separate when the wiring member 3 is pulled upward and the flexible substrate 1 is pulled downward.
When the bonding strength of the soldering junction 4 is low, the wiring member 3 and the flexible substrate 1 are separated by breakage at the position 5 indicated by a dashed line, such as the interior of the soldering junction 4 as shown in FIG. 11. That is, the separation occurs due to the breakage of the soldering junction 4. In contrast, when the bonding strength of the soldering junction 4 is high, the copper bump 3c is separated from the resin film 3a. When a plurality of samples in which the flexible substrates 1 and the wiring members 3 are bonded are pulled upward and downward, respectively, the bonding strength of the soldering junction 4 can be numerically represented. The numerical valve is calculated by comparing the samples which are separated by breakage at position 5 indicated by the dashed line in the interiors of the soldering junctions 4 to the total number of samples.
According to research conducted by the inventors, it was understood that the reason for insufficient bonding strength of the soldering junction 4 could be explained as follows. Since the Au of the Au film 2c melts into the solder when the thickness E1 at the soldering junction 4 is too great, compounds of Au and Sn generate. When compounds of Au and Sn are formed in the soldering junction 4, the compounds precipitate in the solder or on the surface thereof, and cracks in the solder readily occur.
In soldering junctions 4 of conventional bonding pads 2, the observed Au content is approximately 14 percent by weight and the, amounts of the compounds of Au and Sn in the soldering junction 4 are larger. As a result, cracks in the solder tend to readily occur. Consequently, the ratio of samples in which the soldering junctions 4 are broken at the position 5 indicated by the dashed line to the total samples reaches 5 percent.
Furthermore, in the conventional junction assembly, the thickness E1 of the Au film 2c at the surface of the bonding pad 2, as shown in FIG. 10, is so large that Au melts into the solder at the junction boundary between the soldering junction 4 and the bonding pad 2. As a result spread H1 of a solder fillet at the soldering junction 4 is suppressed. Consequently, separation at the boundary between the soldering junction 4 and the bonding pad 2 readily occurs.
The junction assembly of the bonding pad and the copper bump are used in, for example, a magnetic head device, as shown in FIG. 9.
The magnetic head is provided with a load beam 31 in which a base portion is a fixing edge 31a and a front portion is a supporting edge 31b, a flexure 32 provided at the supporting edge 31b of the load beam 31, a magnetic head body 34 mounted on the flexure 32, and a flexible substrate 33 laminated on the flexure 32.
A bonding pad 35 is formed at one edge of the flexible substrate 33.
A copper bump 37 covered by a solder layer and formed at a wiring member 36 extending from the body side of the magnetic head device is brought into contact with the bonding pad 35 and is sandwiched between a heater and a holding plate, so that a soldering junction 4 similar to that shown in FIG. 3 is formed.
In magnetic head devices and the like, since mechanical vibration is frequently generated and mechanical force is frequently applied to soldering junctions, the possibility of the generation of connection failure during use is high when a junction assembly of the conventional bonding pad and the copper bump are used.
In order to solve the problems in the conventional art described above, objects of the present invention are to provide a junction assembly of a bonding pad and a bump, which can prevent decrease in bonding strength of a soldering junction, and to provide a magnetic head device utilizing the junction assembly. In the present invention, the amount of Au in the bonding pad precipitated in the soldering junction can be suppressed, and the dimension of a spread of a solder fillet at the soldering junction can be increased.
A junction assembly according to the present invention comprises a substrate having a conductive pattern and a bonding pad formed thereon, the bonding pad being electrically connected with the conductive pattern and being provided with an Au film on the surface thereof, and a wiring member having a conductive pattern and a bump formed thereon. The bump is electrically connected with the conductive pattern, in which the bonding pad and the bump are bonded to each other by soldering. In the junction assembly described above, in a state in which the bonding pad is bonded to the bump, the thickness of the Au film is determined so that the Au content is not more than 9 percent by weight in a soldering junction between the bonding pad and the bump.
When the thickness of the Au film on the surface of the bonding pad is appropriately determined, the Au content in the soldering junction, which was conventionally approximately 14 percent by weight, can be decreased, the precipitated amounts of compounds of Au and Sn in the solder or on the surface thereof can be decreased, and a cracking of the solder becomes unlikely. In addition, when the Au film is thinned, a phenomenon of Au which melts into the soldering junction at the boundary thereof with the bonding pad can be suppressed, and decrease in wettability of the solder on the surface of the bonding pad can therefore be prevented. Accordingly, the spread of the solder fillet is larger, and separation between the solder and the bonding pad therefore becomes difficult.
In the case in which the Au content in the soldering junction is not more than 9 percent by weight, when the wiring member and the flexible substrate which are bonded to each other by soldering are pulled upward and downward, respectively, the ratio of the number of samples in which the wiring member and the flexible substrate are separated in the soldering junction can be reduced to not more than 1 percent of the total number of the sample. The ratio can be considerably decreased, compared to 5 percent that was obtained in measurements using conventional bonding pads.
More preferably, the Au content in the soldering junction is not more than 7 percent by weight. In the case in which the Au content is not more than 7 percent by weight, when the wiring member and the flexible substrate which are bonded to each other by soldering are pulled upward and downward, respectively, the ratio of the number of samples in which the wiring member and the flexible substrate are separated in the soldering junction can be reduced to 0 percent of the total number of the samples.
As methods for decreasing the Au content in the soldering junction, according to the present invention, the thickness of the Au film of the bonding pad is made to be thinner relative to the bump dimensions. In contrast, when the Au film of the bonding pad is made to be too thin, an Au film having an even thickness thereof cannot be formed, and for example, holes form in the Au film. Consequently, variation in wettability in soldering may occur, and decrease of the wettability due to oxidation of the Ni film beneath the Au film may occur.
Accordingly, in order to decrease the Au content and to prevent the generation of Au film defects, when the diameter A2 of the bump is set to be 1, the thickness E2 of the Au film at the surface of the bonding pad is preferably 0.34xc3x9710xe2x88x923 to 2.41xc3x9710xe2x88x923.
When the dimensions are set as described above, the Au content in the soldering junction can be reduced to not more than 9 percent by weight. In addition, formation of irregularities in the thickness of the Au film due to the Au film being too thin can be avoided.
More preferably, when the diameter A2 of the bump is set to be 1, the thickness E2 of the Au film at the surface of the bonding pad is 0.34xc3x9710xe2x88x923 to 1.72xc3x9710xe2x88x923.
When the dimensions are set as described above, the Au content in the soldering junction can be reduced to not more than 7 percent by weight. In addition, formation of irregularities in the thickness of the Au film due to the Au film being too thin can be avoided.
In order to prevent the formation of irregularities in the thickness of the Au film due to the Au film being too thin, in the present invention, when the diameter A2 of the bump is set to be 1, the thickness E2 of the Au film at the surface of the bonding pad is determined to be not less than 0.34xc3x9710xe2x88x923. In the conditions described above, the Au content in the soldering junction is not less than 1 percent by weight. Accordingly, the preferable Au content in the soldering junction is between 1 to 9 percent by weight, and more preferably, 1 to 7 percent by weight. In the case in which the diameter A2 of the bump is set to be 1, as described above, an opening diameter B2 of the wiring member into which the bump penetrates is 0.62, the bump thickness C2 from the surface of the wiring member is 0.14 to 0.31, the solder thickness D2 on the surface of the bump is 0.07 to 0.12, and the planar dimensions of the bonding pad is 1.90xc3x971.72.
The bonding pad is preferably formed by sequentially depositing a Cu film, an Ni film covering the Cu film, and the Au film covering the Ni film.
When the Ni film covers the Cu film, oxidation of the Cu film is prevented, and wettability with the solder is improved when the Au film covers the Ni film.
In addition, the magnetic head device according to the present invention comprises a load beam fixed at the base portion thereof, a flexure provided at the front portion of the load beam, a magnetic head body mounted on the flexure, a substrate having a conductive pattern connected to the magnetic head body, and a wiring member connected to the substrate, in which the substrate and the wiring member are bonded to each other by one of the junction assemblies described above.
When a magnetic head device is manufactured by using the junction assembly according to the present invention, connection failures at the soldering junction of the bonding pad are unlikely, and a superior quality magnetic head device can be manufactured.